Today's wavelength division multiplexing (WDM: wavelength division multiplexing) communication method using an optical fiber is an electro-optic hybrid system, and a technology is advancing to provide an all-optical network in which signals are processed directly from light. A variable wavelength filter (tunable filter), which makes it possible to select a wavelength, is thought to play an important role in this optical network. A variable wavelength filter (tunable filter) finds applications, for example, in a dynamic add/drop multiplexer or a wavelength router.
Conventionally, various methods have been proposed as a variable wavelength filter (tunable filter) such as a method that mechanically controls the optical path length (MEMS: microelectromechanical system), a Mach-Zehnder type filter that combines a light guide with the thermo-optic effect, and a filter that uses acoustooptics. References that describe those method are, for example, V. M. Bright “Selected Papers On Optical MEMS”, Vol. MS153, SPIE, 1999 and H. T. Mouftah and J. M. H. Elmirghani, “Photonic Switching Technology”, IEEE, 1999. Among various methods, a liquid crystal variable wavelength filter (tunable filter) is expected as a variable wavelength filter (tunable filter) applicable to an optical network because it has not mechanical moving parts and consumes less power. The following describes a Fabry-Perot type filter, in which a liquid crystal layer is used as the cavity, as a typical example.
The references to the above-described liquid crystal Fabry-Perot type filter are, for example, K. Hirabayashi, H. Tsuda, and T. Kurokawa, J Lightwave Technol., vol. 11, No. 12, pp. 2033–2043, 1993. FIG. 22 shows a cross sectional diagram of a basic liquid crystal Fabry-Perot filter. A liquid crystal Fabry-Perot filter 1000 comprises a cavity layer 1003 held between a first dielectric multi-layered film mirror 1017 and a second dielectric multi-layered film mirror 1019 and filled with nematic liquid crystal material 1001. The nematic liquid crystal material 1001 is aligned by a first alignment film 1013 and a second alignment film 1015 so that it is aligned parallel to the film surfaces with pretilt in the cross sectional diagram in FIG. 22.
At this time, anisotropy is induced on the surface of the first alignment film 1013 and the second alignment film 1015 by rubbing. In addition, to allow the cavity layer 1003 to have a predetermined gap, a spacer 1021 is provided to fix a first filter substrate 1005 and a second filter substrate 1007. And, a first transparent conductive film 1009 and a second transparent conductive film 1011 are formed to apply an electric field to the nematic liquid crystal material 1001.
In the liquid crystal Fabry-Perot filter 1000, the cavity layer 1003 configures a resonator and the refractive index of the cavity layer 1003 determines the optical path length. The liquid crystal Fabry-Perot filter 1000 changes the refractive index of the cavity layer 1003 and changes the product of this refractive index and the cavity layer to change the resonant wavelength. The resonant wavelength λm of the liquid crystal. Fabry-Perot filter 1000 is given as follows.λm=2neff(V)·d/m  (1)where, neff(V) is the effective extraordinary refractive index of the cavity layer 1003 and is the function of the applied voltage V. d is the cavity gap, and m is an integer.
In FIG. 22, out of an incoming linearly polarized beam 1031 that enters vertically from above and that is parallel to the cross sectional diagram, only the light with the wavelength corresponding to the resonant wavelength λm, shown in equation (1), transmits through the liquid crystal Fabry-Perot filter 1000 as an outgoing linearly polarized light 1033. The effective extraordinary refractive index neff(0) with no electric field applied to the nematic liquid crystal material 1001 is a constant value in the cavity layer 1003, as shown below, when pre-tilt angle of a liquid crystal director 1041 is θ0.neff(0)=(sin2 θ0/n02+cos2 θ0/ne2)−1/2  (2)where n0 is an ordinary refractive index and ne is an extraordinary refractive index.
When an electric field is applied to the nematic liquid crystal material 1001, the tilt angle θp becomes a large value in the central part in the thickness direction of the cavity layer 1003 according to the applied voltage. This tilt angle θp approaches θ0 as it gets near to the first alignment film 1013 and the second alignment film 1015. Therefore, when the electric field V is applied to the nematic liquid crystal material 1001, the average of the effective extraordinary refractive index neff(V) of the cavity layer 1003 in the thickness direction becomes a smaller value as compared with that when no electric field is applied. As shown in equation (1), the value of the resonant wavelength λm shifts to the shorter wavelength side when compared using the resonant wavelength λm of the same order m. Thus, the liquid crystal Fabry-Perot filter 1000 capable of selectively transmitting a predetermined wavelength can be used as a tunable filter.
However, because the cavity layer 1003 is configured as a liquid crystal cell in the liquid crystal Fabry-Perot filter 1000 such as the one shown in FIG. 22, the cavity layer is a single layer in structure. Therefore, the relation between the transmittance and the wavelength of the liquid crystal Fabry-Perot filter is as shown by a characteristic 2201 in FIG. 23. The characteristic 2201 shows a transmission band characteristic that have the low isolation characteristic of the stop band near the transmission band and that have a steep peak. On the other hand, the ideal characteristic of a variable length filter (tunable filter) is that the isolation characteristic of the stop band near the transmission band is high and that the transmission band characteristic is flat (flat top). A characteristic 2203 in FIG. 23 shows an example of characteristic ideal for 1.6 nm interval (200 G Hz).
As described above, a liquid crystal Fabry-Perot filter has the characteristic 2201 that is significantly different from the characteristic 2203. Those characteristics are a reason that a variable length filter (tunable filter) using a liquid crystal Fabry-Perot filter is not applicable to a dense wavelength division multiplexing (WDM) communication system.
In addition, because the cavity layer 1003 is formed by the nematic liquid crystal material 1001 in FIG. 22, the dielectric multi-layered film mirrors 1017 and 1019 must be formed within the liquid crystal cell. The dielectric multi-layered film mirrors 1017 and 1019 are configured by a film stack of optically quarter wavelength films using, for example, tantalum pentoxide (Ta2O5) as a highly refractive material and silicon dioxide (SiO2) as a low refractive material. For a 1.55 μm band used for optical communication, the dielectric multi-layered film mirrors 1017 and 1019 must be several microns thick and, therefore, the liquid crystal cell fabrication process, such as the gap control and electrode formation of the liquid crystal Fabry-Perot filter 1000, becomes difficult.
Therefore, it is an object of the present invention to provide a liquid crystal variable wavelength filter unit and its driving method that simplifies the liquid crystal cell structure and that is applicable to high-grade optical fiber communication.